1. Field of the Invention
The present invention generally relates to a device for detecting defects of a magnetic head or heads, and more particularly to a device by which the quality of the magnetic head is judged based on a level of a back electromotive voltage of the magnetic head due to an inputted write switching current.
2. Prior Art
First, description will be given with respect to a general control circuit of the device for detecting defects of the magnetic heads in conjunction with FIG. 1.
In FIG. 1 a center tap voltage VCT is applied between coils 1a and 1b of a magnetic head 1, and switching transistors Q1 and Q2 are respectively connected to these coils 1a and 1b. Write signals are supplied to base terminals T1 and T2 of the transistors Q1 and Q2. The levels of these write signals are set inverted each other so that the base terminal T1 will be turned on when the base terminal T2 will be turned off. For example, when the write signal for the base terminal T1 is ON, the transistor Q1 is turned on. Hence, the write switching current due to the center tap voltage VCT flows through a circuit consisting of the coil 1a, collector and emitter of the transistor Q1 and a constant current source 2. Thus, a magnetic recording is executed on media (i.e., magnetic recording media). Just after a down or lagging edge of the write pulse (i.e., a falling edge of the write switching current), a back electromotive voltage is produced at the coil 1a. Meanwhile, 3 designates a processing portion of a read signal of the magnetic head 1, and this processing portion 3 is constituted by a differential amplifier. In case of multi-channel heads, the above-mentioned control circuit is provided for each magnetic head, and all control circuits are connected to the constant current source 2.
As shown in FIG. 1, a detecting circuit 4 for detecting defects of the magnetic heads is directly connected to the coils 1a and 1b via respective diodes D1 and D2. Therefore, the back electromotive voltages produced at the coils 1a and 1b are normally inputted to the detecting circuit 4. In the normal case, such back electromotive voltage is produced just after the down edge of the write pulse. However, in some cases, the magnetic head 1 breaks down due to breaking of the coils 1a and 1b, hence, the back electromotive voltage is prevented from being produced. In such cases, the detecting circuit 4 outputs a detection signal representing that defects of the magnetic head 1 are detected. As disclosed in Japanese Patent Laid-Open Publication No. 62-38505, the detecting circuit 4 can be constructed by use of a charging circuit and an amplifier circuit. More specifically, a capacitor is charged by intermittently produced back electromotive voltage and the potential of such capacitor is maintained at a level higher than a predetermined level in the normal case. On the other hand, when the back electromotive voltage is not produced so that the potential of the capacitor becomes lower than the predetermined level, the detecting circuit 4 outputs the detection signal. Such detection signal outputted from the detecting circuit 4 is used as a signal for urgently stopping a writing operation of the magnetic head in order to protect recorded information of the magnetic recording medium.
As described above, since the detecting circuit 4 is normally connected with the coils 1a and 1b, the following problem will be incurred.
The detecting circuit 4 must input noise which is absorbed by the magnetic head 1. In order to respond to a high density recording, a thin film head having low impedance is used. However, since such thin film head produces less back electromotive voltage, it is difficult to discriminate the back electromotive voltage and the noise, which causes an error in operation of the detecting circuit 4.